Unattended Solar Li-Ion Recharging Safety

[grizzlyjere]

Hi,

I'm looking into developing a roof mounted wireless weather station.  To power it, I'd like to use a solar panel and small LiPo battery. For charging I'd use a pre-built solar charging module from Adafruit or Sparkfun since I don't trust myself to develop one from scratch just yet.  I'd ensure there is a temperature sensor on the battery to shut everything down if it begins to overheat.

My concern with this is safety during the summer heat (95F+) -- how safe is a LiPo is sitting on a hot roof (even if I shade it)?  I recently had a name brand laptop battery fail (it was an older model, but still) and while it didn't cause a fire, the battery did swell significantly.  So I'm more concerned about this than I might normally be.

Am I too paranoid?  Is there a safer way to do this?  Should I be physically isolating the battery (shaded metal box with vent holes and a fan)?
                                               
Thanks for helping a beginner out.

[DavidMenting]

LiPo is definitely not the safest type of battery to use in this situation. Make sure you don't overcharge it by selecting a conservative charge cutoff voltage (like 4.1 or 4.0V) or a higher charge cutoff current. The charger pcbs often have a provision to connect a temperature sensor that you locate near the battery. The charger will then fail to charge if battery temperatures are too high or low.

You might want to switch to LiFePo4 batteries, as they are more stable and more tolerant to abuse. In fact, you can charge them by supplying a constant 3.45V per cell or by directly hook a well-matched (=4-5V) solar panel to the battery through a Schottky diode. You will need to make something that disconnects the solar panel as soon as the battery gets full, however.

[retrolefty]

I'll be interested in the responses also. Most applications like this use gel lead-acid battery packs, although I don't know if they have a better operating temp spec then Li batteries. I know the high performance Li-Po packs the R/C hobby uses are run very very hot on short 10 min total discharges.

[KL27x]

I am pretty anal with my li ion batteries. I keep my li ion tools indoors. No way I leave them in the garage, let alone a hot roof. But my concern is longevity, not spontaneous combustion.

Lead acid are really good with heat.

Maybe you could use a longer wire and some filter caps, and keep the battery somewhere cooler.

[grizzlyjere]

Thanks for the feedback everyone.  I wasn't aware of LiFePo4 batteries and hadn't considered a lead acid battery for this purpose.

[DavidMenting]

When comparing please take into account that you generally don't want to run your lead acid battery down too far. LiFePo4 batteries don't mind being run down flat, so in general you would only need a LiFePo4 battery with half the capacity of a lead acid battery in the same situation. Also, the charge/discharge efficiency of LiFePo4 is a lot higher than lead acid's.

However, a protected lipo or li-ion cell that is kept within limits should be perfectly fine. The cylindrical 18650 cells are fairly robust and easy to find.

Do you know the current draw of the weather station? What battery capacity and voltage are you looking for?

[amyk]

LiFePo4 batteries don't mind being run down flat

Not true. Phosphate has higher endurance than cobalt but still should not be discharged all the way - 2V/cell is the usually recommended absolute minimum. They are fully charged at 3.6V and stay around 3.2V for most of their discharge.

[DavidMenting]

You're right, discharging all the way down to 0V is not a good idea. With 'running down flat' I meant going all the way down to 100% depth of discharge, which like you said is often stated at 2V/cell. Lead acid batteries really don't like 100% DoD.

[The Magic Rabbit]

The best type of cell for this is lead acid based. They can take a continuous trickle charge, so can be coupled to a Solar Panel easily, and are very durable. They tend to be heavy, which is really behind why people don't like them in tools etc.  In a static application they're unbeatable, hence why most Solar systems tend to use them.

An alternative would be NiCd, which is better than NiMH in this kind of application.

Lead Acid batteries are available in 6V and 12V varieties, so pick whichever suits you best.

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